つくづく・熟・熟熟・熟々tsukuzuku[3]nakadaka (mid-high)

1. adverb (with と)deeply; keenly; severely; completely; utterly
   彼かれは、自分じぶんの不幸ふこうをつくづく考かんがえた。↗

   He brooded over his misfortunes.
2. adverb (with と)intently; carefully; closely; attentively

• SynonymじっくりNeutralMeans doing something unhurriedly, taking time to be thorough; the nuance is calm and leisurely. It has none of the bitter, keen emotion of 「つくづく」.
• SynonymしみじみNeutralAlso expresses deep, heartfelt feeling, as in 「しみじみ思う」; close to 「つくづく」 but gentler. 「つくづく」 often pairs with negative feelings like disgust or weariness.
• SynonymとくとWrittenMeans doing something carefully and fully, often with 「見る」 or 「考える」; the tone is solemn and old-fashioned. Unlike 「つくづく」, it carries no keen emotional realization.
• SynonymとっくりCasualColloquial but dated; means thinking or doing something thoroughly, as in 「とっくり考える」. Rarely used now, and lacks the keen emotion of 「つくづく」.
• SynonymつぶさにWrittenWritten style; means examining or reporting in full detail, item by item, as in 「つぶさに調べる」. It stresses leaving nothing out; 「つくづく」 stresses deep thought and feeling.
• Synonym骨の髄までほねのずいまでNeutralA set phrase meaning 'to the marrow', utterly and completely, as in 「骨の髄まで染みる」. More hyperbolic than 「つくづく」 in degree.
• Synonym深くふかくNeutralSimply 'deeply'; very versatile, as in 「深く感謝する」. It lacks the sense of dwelling repeatedly on something or feeling it keenly that 「つくづく」 has.
• Related慎重しんちょうNeutralA na-adjective/noun meaning careful and prudent; it describes acting cautiously. 「つくづく」 is about deep realization, so the two cannot replace each other.
• Related心してこころしてWrittenWritten; means doing something with care and vigilance, as in 「心して聞け」. It stresses caution and alertness, with no link to the deep emotion of 「つくづく」.
• Related大切にたいせつにNeutralMeans treating something as precious, handling it with care, as in 「大切に使う」. It is about valuing the object, not the deep realization of 「つくづく」.

Use 「つくづく」 for deep, keen realization or an emotion you cannot suppress, as in 「つくづく嫌になる」. For doing something carefully over time say 「じっくり」; for milder, wistful feeling say 「しみじみ」; for exhaustive detail say 「つぶさに」.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. じっくり。
2. 身みにしみて。ほとほと。